Method and apparatus for chemical analysis

ABSTRACT

A method and apparatus for determining the concentration of a component in a sample, i.e. the concentration of urea in biological fluids, such as blood serum, wherein the sample, upon being introduced into solution with a reagent, reacts therewith, causing a continuing change in a characteristic of the solution, and wherein the rate of the reaction is indicative of the concentration of the component in the sample. A sensor is provided for monitoring the characteristic of the solution and for generating a first electrical output signal proportional thereto. Differentiator circuit means are provided for producing a second electrical signal proportional to the time derivative of the first signal, the time derivative signal being indicative of the concentration of the component in the sample.

United States Patent [1 1 Paulson et al.

[ Oct. 16, 1973 METHOD AND APPARATUS FOR CHEMICAL ANALYSIS [75]Inventors: Gerald L. Paulson, Anaheim; Robert A. Ray, Fullerton, both ofCalif.

[73] Assignee: Beckman Instruments, Inc.,

Fullerton, Calif.

[22] Filed: Aug. 6, 1971 [21] App]. No.: 169,687

[52] US. Cl. 23/230 R, 23/253 R, l95/103.5 R, 195/127, 324/30 B [51]Int. CL... Cl2k l/l0, GOln 27/06, GOln 33/16 [58] Field of Search 23/230R, 253 R; 195/1035, 127; 324/30 B, 30; 204/195 [56] References CitedUNITED STATES PATENTS 3,458,287 7/1969 Gross et al. 23/230 B 3,635,681l/l972 Rogers 23/253 R Primary ExaminerRobert M. Reese Att0rney--PhilipM. Hinderstein [5 7] ABSTRACT A method and apparatus for determining theconcentration of a component in a sample, i.e. the concentration of ureain biological fluids, such as blood serum, wherein the sample, uponbeing introduced into solution with a reagent, reacts therewith, causinga continuing change in a characteristic of the solution, and wherein therate of the reaction is indicative of the concentration of the componentin the sample. A sensor is provided for monitoring the characteristic ofthe solution and for generating a first electrical output signalproportional thereto. Differentiator circuit means are provided forproducing a second electrical signal proportional to the time derivativeof the first signal, the time derivative signal being indicative of theconcentration of the component in the sample.

32 Claims, 11 Drawing Figures PATENTEDUCT 15 I975 SHEET 2 OF 2 ATTORNEYMETHOD AND APPARATUS FOR CHEMICAL ANALYSIS According to the presentinvention, a large, instantaneous change in the characteristic of thesolution being measured also takes place when the sample is added.Therefore, means are provided for measuring the value of the timederivative signal after a predetermined, fixed time interval fromintroduction of the sample into the reagent so as to eliminate theeffect of the instantaneous change in the characteristic of the solutionand to permit thorough mixing of the sample with the reagent.

BACKGROUND OF THE INVENTION 5 Field of the Invention The presentinvention relates to a method and apparatus for chemical analysis and,more particularly, to a method and apparatus for the quantitativedetermination of the concentration of substances which are reactive withenzymes.

Description of the Prior Art One general area within the field of thisinvention is the chemical analysis of biological substances to determinethe chemical composition thereof. For example, a common procedure is todetermine the concentration of glucosein blood or urine since theconcentration of glucose in these body fluids is indicative of theoperation of various fundamental body functions. Another commonprocedure is to determine the concentration of urea in blood serum sincethe concentration of urea in this body fluid is indicative of theoperation of the kidneys.

Most available analyzing systems for determining the chemicalcomposition of biological substances rely on colorimetric analysis. Forexample, one known technique for the enzymatic assay of glucose in bloodand urine relies on the oxidation of the glucose in blood with theenzyme glucose-oxidase to produce hydrogen peroxide and gluconic acid. Apresently available chemical analyzer relies on the spectrophotometricresponse of the color reaction between hydrogen peroxide, peroxidase anda chromogen. Another example would be in the determination of urea inblood by the reaction of urea with the enzyme urease to produce ammoniumcarbonate and using colorimetric techniques for determining theintensity of the product of the reaction.

While such colorimetric chemical analyzing systems are capable ofproducing accurate indications of the concentration of a component in asample, there are several problems associated therewith. In the firstinstance, most colorimetric techniques are subject to large disturbancesand interferences which may provide grossly inaccurate indications. Forexample, in the enzymatic assay of glucose in blood and urine by theoxidation of glucose with glucose-oxidase, the strong oxidizing agent,hydrogen peroxide, can react with other reducible substances and otherimpurities interfere with the peroxide-peroxidase reaction causing alossin specificity and accuracy. In addition, availabe colorimetric andanalyzing systems require measurement of the intensity of the color ofthe product at the completion of the reaction. Accordingly, the analysisis time consuming. In addition, the assay often cannot be conductedwithout deproteinization of the blood samples or prepurification ofurine samples.

With respect to some enzymatic reactions, it has been proposed to useconductivity measurements in order to determine the concentration of acomponent in a sample. More specifically, in certain enzymaticreactions, a change occurs from a non-ionic to an ionic species or froman ionic to a non-ionic species. In such cases, the AC conductance ofthe medium serves as a direct measure of the extent of the reaction andthe rate of change of AC conductance measures the rate of the reaction.Since the rate of reaction is directly proportional to theconcentrations of certain reactants, such as the enzyme and thesubstrate, the concentrations of these species can be monitored bymeasuring the rate of change of AC conductance.

An example of this type of reaction is the reaction that occurs whenblood, containing urea, is mixed with the enzyme urease. The non-ionicurea in the serum reacts with the enzyme urease to form ionic ammonium.carbonate. The rate at which ammonium carbonate is formed isproportional to the quantity of urea in the serum .sample. Sinceammonium carbonate is ionic, the AC conductivity of the solution willchange at a rate proportional to the quantity of urea present.

U. S. Pat. No. 3,421,982 to F. C. Schultz et al for Enzymatic Analysisproposes a system for measuring this change in AC conductivity. Thesystem of Schultz et al employs conventional conductance electrodes andheretofore conventional levels of urease to provide a constant rate ofchange of concentration with time. Schultz et al indicate that theconversion of the substrate proceeds for several minutes but that therate of change of conductivity in the first minute is essentiallylinear. These conditions are required so that a twopoint kinetic methodcan meaningfully be used, with an adequate approximation to the rate ofreaction being .determined by measuring the finite change in conductanceoccurring over a fixed time interval within the linear portion of thereaction. Mathematically, this is a measurement of 66 C/At where AC isthe change in conductance during the fixed time interval, At, which isapproximately one minute. As a result, the system of Schultz et al iscumbersome, time consuming and subject to inaccuracies.

One system for solving not only the problems inherent in colorimetricanalysis systems but also in the conductimetric system of Schultz et alis disclosed in copending application Ser. No. 618,859, filed Feb. 27,1967 in the name of James C; Stemberg for Rate Sensing Batch Analyzerand assigned to Beckman Instruments, Inc., the assignee of the presentapplication. The analyzer disclosed therein I provides a convenientmethod for rapidly determining quantitative information concerning aseries of chemical, and especially biological samples. That analyzerdetermines the concentration of substances'reactive with enzymes rapidlyand accurately and uses small sample sizes. The analyzer of thatapplication relies on the measurement of true instantaneous rate ofreaction at very early stages of the reaction, before much reactant isconsumed. The recorded rate signal results in a sharply defined peakcorresponding to apparent maximum rate which is directly proportional toinitial concentration. The apparent maximum rate is obtained in arelatively short time interval, thus saving analysis time and permittingmore samples to be run in the same time interval. As applied to thedirect monitoring of oxygen consumed in a glucose oxidase-glucosereaction, the invention does not require preliminary purification ordeproteinization of blood or urine samples, gives highly accurateresults on an absolute basis and is insensitive to many impurities knownto interfere with many other analytical procedures.

While such Rate Sensing Batch Analyzer solves the problems of the priorart discussed hereinbefore, it has been found that such analyzer is notideally suited for many enzyme reactions. For example, the Sternberganalyzer determines the quantity of glucose in blood or urine by usingan oxygen sensor to measure the rate of oxidation of glucose withglucose-oxidase to produce hydrogen peroxide and gluconic acid. Thereaction may be controlled so that there is no initial change in oxygenlevel when the sample is introduced into solution with the reagent. Onthe other hand, when determining the concentration of urea in bloodserum by reacting the serum with urease to form ammonium carbonate,obviously an oxygen sensor cannot be utilized to monitor the reaction.If a conductivity sensor is utilized for measuring change in conductanceof the solution, problems are encountered in measuring the maxi- I mumvalue of the time derivative of the output of the sensor, as taught andclaimed in said copending application. This is because blood serumitself is conductive and there is a large conductivity change in thesolution when the serum is added to the reagent. This instantaneous jumpin solution conductivity generates an apparent maximum value of timerate of change of conductivity which approaches infinity such that ameaningful output cannot be obtained.

SUMMARY or THE INVENTION According to the present invention, there isprovided a method and apparatus for chemical analysis which not onlysolves the problems of the prior art solved by the before-mentioned RateSensing Batch Analyzer, but is applicable to a wider variety of enzymereactions. The present method and apparatus is capable of rapidlydetermining the concentration of a component in a sample, such asbiological fluids. The present apparatus is rapidly set up and put intooperation, makes the determinations rapidly and accurately, uses a smallsample size, and measures true concentration.

The present method and apparatus relies on the measurement of trueinstantaneous rate at a very early stage of a reaction before thereactant is consumed and, even with gaseous reactants, the reactions canbe open to the atmosphere since the indicative data is collected beforeback diffusion of gas into the solution can influence the results. Thepresent method and. apparatus recognizes that introduction of the sampleinto solution with the reagent may cause an instantaneous change in thecharacteristic of the solution which is being measured. Accordingly, thepresent system inhibits measurement of the rate of change signal duringa predetermined, fixed time interval starting with introduction of thesample into the reagent, which time interval is sufficient to eliminatethe effect of the instantaneous change in the solution as well as topermit thorough mixing of the sample with the reagent. lmmediately afterthe termination of the fixed time interval, the present system measuresthe value of the rate of change of the reaction.

Briefly, the present invention contemplates a method and apparatus fordetermining the concentration of a component in a sample, i.e. theconcentration of urea in biological fluids, such as blood serum, whereinthe sample, upon being introduced into solution with a reagent, reactstherewith, causing a continuing change in a characteristic of thesolution, and wherein the rate of the reaction is indicative of theconcentration of the component in the sample. A sensor is provided formonitoring the characteristic of the solution and for generating a firstelectrical output signal proportional thereto. Differentiator circuitmeans are provided for producing a second electrical signal proportionalto the time derivative of the first signal, the time derivative signalbeing indicative of the concentration of the component in the sample.

According to the present invention, a large, instantaneous change in thecharacteristic of the solution being measured also takes place when thesample is added. Therefore, means are provided for measuring the valueof the time derivative signal after a predetermined, fixed time intervalfrom introduction of the sample into the reagent so as to eliminate theeffect of the instantaneous change in the characteristic of the solutionand topermit thorough mixing of the sample with the reagent. Thus, thepresent invention is capable of handling a large instantaneous change inthe solution which is of little or no interest followed by a smaller andslower change which is indicative of the concentration of the componentof interest.

It is therefore an objectof the present invention to provide a novelmethod and apparatus for chemical analysis.

It is a further object of the present invention to provide a method andapparatus for the quantitative determination of the concentration ofsubstances which are reactive with enzymes.

It is a still further object of the present invention to determine theconcentration of a substance reactive with an enzyme by measuring thevalue of the rate of the reaction after a predetermined, fixed timeinterval from introduction of the substance into the enzyme.

It is another object of the present invention to provide a method andapparatus for measuring the concentration of urea in blood serum.

It is still another object of the present invention to measure enzymaticreactions rapidly and accurately with a minimum sample size.

Another object of the present invention is the provision of a novelelectrode for measuring AC conductance.

Still other objects, features and attendant advantages of the presentinvention will become apparent to those skilled in the art from areading of the following detailed description of the preferredembodiments constructed in accordance therewith, taken in conjunctionwith the accompanying drawings, wherein like numerals designate likeparts in the several figures and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph illustrating oxygen(0 concentration as a fimction of time (t) in a glucose oxidaseglucosereaction;

FIG. 2 is a graph illustrating the time derivative of oxygenconcentration (dO /dt) for reaction of FIG. 1;

FIG. 3 is a graph illustrating AC conductivity (C) as a function of time(t) for certain enzyme reactions, such as a urea-urease reaction;

FIGS. 4-6 are graphs illustrating the rate of change of AC conductivity(dC/dt) versus time (t) for the graph of FIG. 3 and showing threealternative methods for measuring the value of the rate of change signalafter a predetermined, fixed time interval from introduction of thesample into the reagent;

FIG. 7 is a simplified block diagram showing a preferred embodiment ofapparatus constructed in accordance with the teachings of the presentinvention;

FIG. 8 is a partial block diagram showing a possible modification to theapparatus of FIG. 7;

FIG. 9 is a view, partly in section, showing a preferred embodiment ofsample cup for use in the apparatus of FIG. 7;

FIG. 10 is a side elevation view of a preferred embodiment of sensor foruse in the apparatus of FIG. 7; and

FIG. 11 is an end elevation view of the sensor of FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The beforementioned copendingapplication of James C. Sternberg discloses a convenient method forrapidly determining the concentration of substances reactive withenzymes by measuring true instantaneous rate of reaction at very earlystages of the reaction. As applied to the determination of the quantityof glucose in blood or urine, such analyzer contemplates measurement ofthe rate of oxidation of glucose with glucoseoxidase to produce hydrogenperoxide and gluconic acid. Measurement is made by positioning an oxygensensor within a sample cup and measuring the rate of change of oxygenconcentration.

Referring now to the drawings and, more particularly, to FIG. 1 thereof,at a time t prior to introduction of the glucose-containing sample intothe oxygenated glucose oxidase, the level ofoxygen 0 may have a value 0Upon introduction of the sample, at time the level of oxygen follows acurve 10 which decreases asymptotically. If the output of the oxygensensor is applied to a differentiating circuit, an electrical signal maybe derived which is the time derivative of the oxygen concentrationsignal and thus proportional to the time rate of change of concentrationof oxygen. With reference to FIG. 2, curves 11, I2 and 13 representthree possible outputs of such differentiating circuit. Morespecifically, upon differentiating the output of the oxygen sensor, thetime derivative increases to a maximum value and then decreases as therate of reaction decreases. The maximum value of the output time rate ofchange signal is directly proportional to the 7 initial concentration ofglucose and provides a convenient, rapid and accurate output signal.

Many other enzymatic reactions are such that introduction of the sampleinto solution with the reagent cuases no instantaneous change in thecharacteristic of I it is desired to measure. For example, it ispossible to determine the concentration of urea in blood serum byreacting the serum with the enzyme urease to form ammonium carbonate.The rate at which ammonium carbonate is formed is proportional to thequantity of urea in the serum sample. Since the serum is initiallynonionic and since ammonium carbonate is ionic, the AC conductivity ofthe solution will change at a rate proportional to the quantity of ureapresent. However, problems are encountered in measuring the maximumvalue of the output of an AC conductance sensor since blood serum itselfis conductive and there is a large conductivity change in the solutionwhen the serum is added to the reagent. Referring now to FIG. 3, curve15 shows the change in AC conductance with time measured by aconductance sensor positioned within a sample cup. At time t when thecup is empty, the AC conductance has a value C O. At time I when thesample cup is filled with the enzyme urease, the AC conductivity jumpsto a value C becuase of the conductivity of the reagent. At time whenthe serum is introduced into the sample cup, there is an immediate jumpin conductivity to a value C because of the conductivity of the bloodserum. Thereafter, the conductivity continues to increase asymptoticallyto a maximum value C the change in conductivity from C to C being causedby the formation of ammonium carbonate.

Referring now to FIG. 4, becuase of the instantaneous jump in solutionconductivity at time t:, the rate of change of AC conductivity dC/dtinitially jumps to infinity, as shown by the dashed curve 16. After timet dC/dt decreases asymptotically along dashed curve 17. However, it willbe appreciated by those skilled in the art that this instantaneous jumpin solution conductivity, at generates an apparent maximum value ofdC/dt which approaches infinity such that a system which measures themaximum value of time rate of change of the sensor output is incapableof providing a useful output signal.

Referring now to FIG. 7, a simplified block diagram of the presentmethod and apparatus for chemical analysis, generally designated 20,includes a sample cup 21 in which the enzyme reaction occurs. Sample cup21 may have any one of many known configurations and includes means forpermitting introduction of the reagent and the sample as well as meansfor insuring thorough mixing of the solution. A preferred embodiment ofsample cup 21 will be described hereinafter with reference to FIG. 9.

Extending into sample cup 21 is a sensor 22 for monitoring acharacteristic of the solution or a component or a product of thereaction and for producing a first electrical output signal on a line 23proportional to such characteristic. Accordingly, sensor 22 may be anytype of known sensor such as the oxygen sensor of the before-mentionedcopending application of Sternberg, such as a spectrophotometric sensoror the like. According to the preferred embodiment of the presentinvention, sensor 22 is a conductivity sensor, of a type to be describedmore fully hereinafter with regard to FIGS. 10 and 11, including firstand second spaced electrodes for sensing the AC conductance of thesolution within sample cup 21.

A constant amplitude AC voltage is applied to one electrode of sensor 22from an oscillator 24. The output of oscillator 24 may be a symetricalwave of any shape, i.e. sinusoidal, square, triangular, etc., having anydesired frequency, depending on circuit parameters, as will be explainedmore fully hereinafter. The change in AC conductivity of the solutionproduces a change in the current on line 23 connected to the otherelectrode of sensor 22, which current is reflected as an amplitudemodulation of the basic frequency signal from oscillator 24.

The amplitude modulated signal from sensor 22 is applied to ademodulator 25 which produces a DC voltage on a line 26 proportional tothe AC conductivity of the solution in cup 21. Line 26 is connected toone fixed terminal 27 of a switch 28 which includes a moveable arm 29.Arm 29 is connected to a suitable display device 30 such as a digitalvoltmeter. Accordingly,,by positioning arm 29 of switch 28 in contactwith terminal 27, the DC voltage proportional to the AC conductivity ofthe solution may be directly read out on display 30. This signal shouldappear as curve 15 in FIG. 3. This feature permits monitoring of theactual AC conductance of the solution in sample cup 21 to determine thevalues of C C C and C The DC voltage proportional to AC conductivity online 26 is also applied to a differentiator circuit 31 which isoperative to produce, on a line 32, a second electrical output signalproportional to the time derivative of the AC conductivity signal online 26. Thus, the electrical signal on line 32 is proportional to thetime rate of change of AC conductance of the solution in sample cup 21and is directly proportional to the concentration of the reactants insample cup 21.

it can therefore be seen that apparatus 20 is useful in monitoring alarge class of enzymatic reactions, such as those where a change occursfrom a non-ionic to an ionic species, or vice-versa, as described morefully in the before-mentioned copending application of Sternberg. Such areaction occurs when blood serum containing urea is reacted with theenzyme urea'se to form ammonium carbonate. As explained previously,since the ureais initially non-ionic and since ammonium carbonate isionic, the AC conductivity of the solution will change, and at a rateproportional to the initial concentration of urea.

Referring again to FIGS. 3 and 4, curve shows the output of demodulator25 on line 26 as a function of time. At time 1 with sample cup 21 empty,the conductance has a value C =0. A measured volume of reagent,containing the enzyme urease, is injected into sample cup 21 at time t,,completely immersing sensor 22. when this occurs, the AC conductivity'online 26 jumps to a value C 1 because of the conductivity of the reagent.A more elaborate discussion of the reagent will be provided hereinafter.A very small volume of sample serum is then introduced into sample cup21 at time and mixed with the reagent. Accordingly, and as shown in FIG.3, at time there is an immediate jump in conductivity to a-value Cbecause of the conductivity of the blood serum. in addition, thenon-ionic urea reacts with the urease to form ammonium carbonate at arate which is proportional to the quantity of urea in the sample.Accordingly, the conductivity continues to increase until a maximumvalue C is reached.

Differentiator 31 provides an output voltage proportional to the rate ofchange of AC conductivity. Becuase of the instantaneous jump in solutionconductivity at time the rate of change of conductivity initially jumpstoward infinity (dotted curve 16), preventing the measurement of maximumvalue of time rate of change. However, according to the presentinvention, the output on line 26 from demodulator 25 is applied to arate sensing circuit 35 which senses the jump in conductivity when theserum sample is injected and which generates an electrical signal onaline 36 indicative of such jump. Alternatively, the output on line 26from demodulator 25 may be applied to a conductivity level sensingcircuit (not shown) which would sense the 5 jump in conductivity whenthe sample is injected and which would also generate an electricalsignal indicative of such jump. in any event, the signal on line 36 isapplied to a time delay circuit 37 which generates, on a line 38, asuitable electrical control signal, a characteristic of which changesafter a predetermined, fixed time interval. The length of this fixedtime interval is chosen based upon several considerations. In the firstinstance, the time interval is selected to be long enough to permit thetransient from the jump in conductivity to disappear sufficiently tomake an accurate measurement of rate of change of conductivity. The timeinterval is also selected to permit elimination of other transients,such as temperature upset and the like. Finally, the fixed time intervalis selected to be long enough to permit thorough mixing of the sampleserum with the reagent. In a preferred embodiment, as describedhereinafter, the change in characteristic of the output of time delay 37occurs approximately 12 seconds after sample introduction.

in any event, according to a first embodiment of the present invention,the output of time delay 37, on line 38, is applied to differentiator 31for inhibiting the operation thereof until the end of the time interval,at time At time after the termination of the time interval, and as shownin FIG. 4, the output 41 of differentiator 31, on line 32, rises to theactual signal level (dotted curve 17) and then falls with the reactionrate. When this occurs, a signal peak 42 is obtained which isproportional to the value of the rate of change signal after apredetermined, fixed time interval from introduction of the sample intothe reagent and is, therefore, proportional to the urea concentration inthe sample. This output from differentiator 31, on line 32, is appliedto a rate measuring circuit 40 which, in this embodiment, senses andholds peak value 42 and applies this value as an output signal over aline 43 to a second fixed terminal 44 of switch 28. Accordingly, bymoving arm 29 of switch 28 into contact with terminal 44, the peaksignal from differentiator 31 may be read out on display 30.

It will be appreciated by those skilled in the art that the operation oftime delay 37, differentiator 31 and rate measuring circuit 40 justdescribed is only one specific manner of affectuating the broaderteaching of the present invention, namely measuring the value of theoutput of differentiator 31 after a predetermined, fixed time intervalfrom introduction of sample into the reagent. In the embodiment shown inFIGS. 4 and 7, the control signal from time delay 37, on line 38, isused to inhibit the operation of differentiator circuit 31 until time2;, whereupon rate measuring circuit 40 measures the maximum value ofthe signal on line 32 immediately thereafter. Other techniques areobviously possible. For example, with reference to FIGS. Sand 8, ratemeasuring circuit 40 may be in the nature of a sample and hold circuitand the control signal from time delay 37, on line 38, may be applied torate measuring circuit 40 to select the time or times for sampling theoutput of differentiator 31. More specifically, time delay 37 may beoperative to generate on a line 48, a second electrical control signal,a characteristic of which changes at a time t, occurring after time 1but prior to time t;,. As shown in FIG. 5, this second control signal online 48 inhibits differentiator 31 from time 1 to time t,, to preventthe disturbance of differentiator 31 in the presence of the large jumpin conductivity at time t When such transient has been eliminated, thesecond control signal on line 48 permits differentiator circuit 31 tobegin operation so that the output thereof rises along curve 49 untilreaching the actual signal level (dotted curve 17) and then falls withthe reaction rate. However, even though differentiator 31 is permittedto start operation at time I it is still desirable to wait until time tomeasure the output of differentiator 31 so as to provide a sufficientamount of time to eliminate the effects discussed previously.Accordingly, the output of time delay 37 on line 38 is applied to ratemeasuring circuit 40 which is activated at time Rate measuring circuit40 measures the instantaneous value 50 of the output of differentiatorcircuit 31, at time 1 and applies such value as an output signal todisplay 30 via switch 28. According to another embodiment of the presentinvention, and as shown in FIG. 6, rate measuring circuit 40 samples thevalue 51 of the signal on line 32 at a time so as to derive the value ofthe signal on line 32 at a predetermined time which need not necessarilycoincide with the apparent rate peak 50.

Referring now to FIG. 9, a preferred embodiment of sample cup includes acylindrical, hollow body 60, forming a chamber 59, the bottom of whichis tapered at 61. The apex of tapered section 61 is-connected to avertical passageway 62 which is connected to a horizontal passageway 63extending entirely through body 60, adjacent the bottom thereof. One end64 of passageway 63 provides an inlet for conducting reagent from asuitable source through passageways 63 and 62 into chamber 59. The otherend 65 of passageway 63 provides a convenient location for emptying thesolution in cup 21. It will be apparent that exit 65 is blocked duringfilling of cup 21 whereas inlet 64 is blocked during draining of cup 21.

Body 60 is open at the upper end thereof, at 66, and may include asuitable collar 67 if desired. The tip 68 of a pipette 69 is adapted tobe extended through the open upper end 66 of body 60 to introduce a verysmall volume of sample, such as serum, into the reagent in chamber 59.In order to insure thorough mixing of the sample with the reagent insample cup 21, sample cup 21 includes a stirrer 70. Stirrer 70 shouldhave a shape similar to that shown in FIG. 9. To prevent the problem ofcoupling a drive element to stirrer 70, stirrer 70 may be magnetized andmay be driven by the rotating magnetic force generated by a rotatingdrive magnet 71 sample, such as serum, is introduced into sample cup 21via pipette 69 where it is mixed with the reagent due to the action ofstirrer 70.

As shown in FIG. 9, body 60 of sample cup 21 may include an opening 75in the side thereof, which is partially threaded, at 76, for receipt ofsensor 22. Any suitable sensor having a pair of electrodes may be usedfor measuring AC conductivity. For example, the beforementioned U. S.Pat. No. 3,421,982 to Shultz et al teaches the use of a pair of parallelelectrodes in a conductimetric system. However, in accordance with theteachings of the present invention, such electrodes should have aspecific construction in order to eliminate many problems that occur inconductance measuring systems. However, before discussingin detail thepreferred embodiment of electrode constructed in accordance with thepresent invention, the following discussion of the problems involved isprovided.

Classical conductance is defined as the reciprocal of the electrical DCresistance and is represented by the 7 equation:

C l/R where C equals conductance and R equals DC resistance. However,the polarization effects of DC systems have required that mostinstruments use an AC voltage to measure this so-called conductance. Infact, an AC system measures the reciprocal impedance Z in accordancewith the equation:

where X is the capacitive reactance due to ions in the solution.Normally the capacitive reactance term X remains quite large untilfrequencies on the order of several megacycles are reached. Sincefrequencies this high are not practical, most conventionalconductimeters have a capacitance balancing circuit built in toaccomodate this problem.

In accordance with the teachings of the present invention, it has beendetermined that the reason for this large capacitive reactance term isthe use of convenconnected by a shaft 72 to a motor 73. In order tosupport stirrer 70 for rotation within body 60 of cup 21,

the lower end of stirrer 70 may be tapered, as at 74, at approximatelythe same angle as tapered section 61 of I chamber 59. By making stirrer70 of some suitable material, such as teflon, the tapered surfaces 74and 61 provide adequate bearing surfaces for stirrer 70. A drainagepassage from cup 21 is provided by slots 74 in the bottom of stirrer 70.A suitable type of stirrer is disclosed in U. s. Pat. No. 3,591,309issued to Robert A. Ray et al and assigned to Beckman Instruments, Inc.

In operation, motor 73 is activated to rotate magnet 71 at any desiredspeed whereby stirrer 70 follows such speed of rotation. A measuredamount of reagent is introduced into chamber 59 of cup 21 via inlet 64and passageways 63 and 62. Thereafter, a small volume of tionalparallel-plate sensors. Referring now to FIGS. 10 and 11, there is showna preferred embodiment of sensor 22 which substantially solves thisproblem. Sensor 22 includes an elongated cylindrical body 80 having adiameter equal to the diameter of the opening 75 in body 60 of samplecup 21. The forward end of body may be threaded at 81 to engage withthreads 76 in opening 75. Body 80 may also include a retaining nut 82which is adapted to be tightened against the outer surface of body 60 ofcup 21. Extending into sample cup 21 is a surface 83. Positioned onsurface 83 are first and second electrodes 84 and 85 which are connectedto leads 86 and 87, respectively, extending through body 80 of sensor22. Leads 86 and 87 may be connected to oscillator 24 and demodulator25, respectively.

According to the teachings of the present invention, the capacitivereactance term may be minimized and, in effect, eliminated, by makingelectrodes 84 and 85 planar and by positioning them coplanar. By sopositioning electrodes 84 and 85, the DC resistance is uneffected butthe capacitance is substantially minimized thereby permitting thecapacitive reactance term X to become very small at a much lowerfrequency.

Surface 83 may be a flat surface and electrodes 84 and 85 may beconductive areas deposited thereon. As

a practical matter, making surface 83 flat and planar is not compatiblewith the cylindrical configuration of the wall of chamber 59 and wouldprevent rapid and thorough mixing of the solution therein and efficientdrainage thereof. Accordingly, and as shown in FIGS. and 11, surface 83is generally curved, having the shape of a segment of a sphere. Withsuch a configuration, electrodes 84 and 85 may be in the shape of halfcircles positioned with the straight sides parallel and spaced apart.With such a configuration, it has been found that the capacitivereactance term X in the reciprocal impedence Z is effectively reduced tozero by increasing the frequency of oscillator 24 to 10 kHz. Of course,the frequency at which the reciprocal impedence plateaus is quitedependent upon the electrode configuration and the 10 kHz value onlycorresponds to the electrode configuration shown in FIGS. 91 1. In anyevent, with such an electrode configuration, the AC impedance becomes avery close approximation to the DC re-- sistance and no capacitancebalancing circuit is required. The output of oscillator 24 may beconnected directly to one'of leads 86 or 37 whereupon the other leadprovides the electrical signal on line 23 for direct connection todemodulator 25.

As explained previously, one of the advantages common to the presentinvention and the Rate Sensing Batch Analyzer of James C. Sternberg isthe provision of convenient methods for rapidly determining quantitativeinformation concerning biological samples. The present analyzer, as wellas that of Sternberg, relies on the measurement of true instantaneousrate of reaction at a very early stage of the reaction, before muchreactant is consumed. By obtaining this rate in a relatively short timeinterval, analysis time is saved permitting more samples to be run inthe same time interval. Accordingly, as applied to the presentinvention, the quantity of enzyme reagent used is large compared tousual methods whereby the reaction proceeds at a relatively rapid rate.The useful reaction is an approximately exponential change ofconductance, as shown in both FIGS. 1 and 3, having a typical timeconstant of 20 seconds. This is diametrically opposed to the teachingsof Schultz et al who use such a small amount of reagent that thereaction proceeds very slowly and can be stated to be approximatelylinear.

Still another factor must be considered in making a conductivitymeasurement as described herein. More specifically, in enzyme reactionsgenerally, the reagent is usually buffered with salts which are highlyconductive so that the pH of the solution remains relatively constant asthe reaction continues. This technique permits the reaction rate toproceed at its maximum potential value. With the present invention, itis apparent that this would be objectionable since it is herein desiredto allow the conductivity of the solution to change, which change andthe rate thereof is measured to determine the concentration of one ofthe components of the reaction. Accordingly, the present inventioncontemplates starting with essentially pure urease, dissolved in water,a low salt preparation having a relatively low initial conductivity.Typically, before the sample is injected, the conductance C,, as in FIG.3, is about 20-25 percent of the final conductance C Again, theconcentration of enzyme reagent relative to the usual concentration ofenzyme is relatively high. When the sample is injected at time theconductance jumps to C, which will have a value in the vicinity of 80percent of the final conductance C Of course, the initial conductance C,of the reagent may have a wide range of values since there still will bea change of conductance during formation of ammonium carbonate. However,because of the inherent difficulty of measuring a small change in alarge signal, it is desirable to keep the initial concentration as smallas possible.

It can therefore be seen that in accordance with the present invention,there is provided a method and apparatus for chemical anaylsis which notonly solves the problems of the prior art solved by the beforementionedRate Sensing Batch Analyzer, but is applicable to a wider variety ofenzyme reactions. The present method and apparatus is capable of rapidlydetermining the concentration of a component in a sample, such as theconcentration of components in biological fluids. The method andapparatus represented by block diagram 20 may be rapidly set up and putinto operation to make quantitative determinations of true concentrationrapidly and accurately, using small sample sizes.

The present method and apparatus relies on the measurement of trueinstantaneous rate at a very early stage of a reaction before thereactant is consumed and during a non-linear portion of the reaction. Onthe other hand, the present invention reconizes that introduction of asample into solution with a reagent may .cause an instantaneous changein the characteristic of the solution which is being measured.Accordingly, the present system inhibits measurement of the rate ofchange signal during a predetermined, fixed time interval starting withintroduction of the sample into the reagent, which time interval issufficient to eliminate the effect of the instantaneous change in thesolution as well as to permit thorough mixing of the sample with thereagent. Immediately after the termination of the fixed time interval,the present invention contemplates measuring a value of the rate ofchange of the reaction. Several specific embodiments have been describedfor accomplishing this result. In addition, the present in ventioncontemplates a novel conductivity sensor for eliminating capacitiveinfluences in a conductance measurement.

While the invention has been described with respect to several physicalembodiments constructed in accordance therewith, it will be apparent tothose skilled in the art that various modifications and improvements maybe made without departing from the scope and spirit of the invention.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrative embodiments, but only by the scopeof the appended claims.

We claim: v

l. A method for determining the concentration of a component in asample, wherein the sample, upon being introduced into solution with areagent, reacts therewith, the rate of the reaction being indicative ofsaid concentration, comprising:

monitoring a characteristic of said solution or a component or productof said reaction which is proportional to said concentration;

. generating an output signal proportional to the time rate of change ofsaid characteristic;

measuring the value of said output signal; and

inhibiting the measurement of the value of said output signal for apredetermined, fixed time interval from introduction of said sample intosaid reagent,

said time interval being sufficient to permit thorough mixing of saidsample with said reagent.

2. A method according to claim 1 wherein said fixed time interval islong enough to permit thorough mixing of said sample with said reagent.

3. In a chemical analyzing system for determining the concentration of acomponent in a sample, wherein said sample, upon being introduced intosolution with a reagent, reacts therewith, the rate of the reactionbeing indicative of said concentration, such system comprising sensormeans for monitoring a characteristic of said solution or a component ora product of said reaction and for producing a first electrical outputsignal porportional thereto, and differentiator circuit means forproducing a second electrical signal proportional to the time derivativeof said first signal, said time derivative signal being indicative ofsaid concentration of said component in said sample, the improvementcomprising:

means for inhibiting operation of said differentiator circuit means fora predetermined, fixed time interval from introduction of said sampleinto said reagent, said time interval being sufficient to permitthorough mixing of said sample with said reagent.

4. In a chemical anaylzing system for determining the concentration of acomponent in a sample, wherein said sample, upon being introduced intosolution with a reagent, reacts therewith, the rate of the reactionbeing indicative of said concentration, such system comprising sensormeans for monitoring a characteristic of said solution or a component ora product of said reaction and for producing a first electrical outputsignal proportional thereto, differentiator circuit means for producinga second electrical signal proportional to the time derivative of saidfirst signal, said time derivative signal being indicative of saidconcentration of said component in said sample, and means responsive tosaid second signal for generating an output signal indicative of thevalue thereof, the improvement comprising:

means for inhibiting the generation of said output signal for apredetermined, fixed time interval intitiated automatically uponintroduction of said sample into said reagent, said time interval beingsufficient to eliminate the effect of transients occurring uponintroduction of said sample into said reagent.

5. In a chemical analyzing system according to claim 4, the improvementwherein said inhibiting means inhibits the operation of saiddifferentiator circuit means during said fixed time interval.

6. In a chemical analyzing system according to claim 4, the improvementwherein said inhibiting means inhibits the operation of said generatingmeans during 6 being indicative of said concentration, such system thetime derivative of said first signal, said tirne deriva-' tive signalbeing indicative of said concentration of said component in said sample,and means for measuring the value of said time derivative signal, theimprovement comprising:

means for inhibiting the measurement of said time derivative signal fora predetermined, fixed time interval from introduction of said sampleinto said reagent, said time interval being sufficient to eliminate theeffect of transients occurring upon introduction of said sample intosaid reagent.

8. In a chemical analyzing system according to claim 7, the improvementwherein said fixed time interval is long enough to permit thoroughmixing of said sample with said reagent.

9. In a chemical analyzing system according to claim 7, the improvementwherein the rate of said reaction is not necessarily linear.

10. In a chemical analyzing system according to claim 7, the improvementwherein said inhibiting means inhibits the operation of saiddifferentiator circuit means during said fixed time interval.

11. In a chemical analyzing system according to claim 7, the improvementwherein said inhibiting means inhibits the operation of said measuringmeans during said fixed time interval.

12. A chemical anaylzer comprising:

means for receiving a sample and a reagent;

sensor means operatively associated with said receiving means formonitoring the concentration of a component or product of the reactionbetween said sample and said reagent and for producing a first outputsignal proportional to said concentration; differentiator circuit meanscoupled to said sensor means and responsive to said first output signalfor producing a second output signal proportional to the time derivativeof said first output signal and thus proportional to the time rate ofchange of concentration of said conponent or product; means coupled tosaid differentiator circuit means for measuring the value of said secondsignal; and

means coupled to said sensor means for inhibiting the measurement of thevalue of said second signal for a predetermined, fixed time intervalfrom introduction of said sample and said reagent into said receivingmeans, said time interval being sufficient to eliminate the effect oftransients occurringupon introduction of said sample and said reagentinto said receiving means.

13. A chemical analyzer according to claim 12 wherein said inhibitingmeans comprises:

timing means responsive to an abrupt change in said first output signalfor producinga control signal, a characteristic of which changes aftersaid predetermined, fixed time interval; and wherein said measuringmeans comprises:

means responsive to said second output signal andoperative upon theoccurrence of said change in said characteristic of said control signalfor determining the value of said second output signal.

14. A chemical anaylzer according to claim 13 wherein said controlsignal is applied to said differentiator circuit means for inhibitingthe operation thereof during said fixed time interval and wherein saidmeasuring means measures the maximum value of said second output signal.

15. A chemical anaylzer according to claim 13 wherein said timing meansproduces a second control signal, a characteristic of which changesbefore the termination of said fixed time interval, wherein said secondcontrol signal is applied to said differentiator circuit means forinhibiting the operation thereof during a first portion of said fixedtime interval, and wherein said first-mentioned control signal isapplied to said measuring means for inhibiting the operation thereofduring said fixed time interval.

16. A chemical analyzer according to claim 12 wherein said sensor meanscomprises:

first and second electrodes extending into said receiving means formonitoring the conductance of the solution therein, wherein:

said second output signal is proportional to the rate of change ofconductance of said solution, and wherein:

said measuring means measures the value of said rate of change ofconductance after said predetermined, fixed time interval. 17. Achemical anaylzer according to claim 16 further comprising:

oscillator means operatively coupled to one of said electrodes of saidsensor means, said oscillator means generating an AC output signal; and

demodulator means operatively coupled to the other of said electrodes ofsaid sensor means, said demodulator means receiving an amplitudemodulated signal and producing a DC signal which comprises said secondoutput signal.

18. A chemical analyzer according to claim 16 wherein said sensor meansincludes a surface which is exposed to said solution, and wherein saidelectrodes comprise first and second conductive areas positioned on saidsurface, said conductive areas being spaced apart.

19. A chemical analyzer according to claim 16 wherein said sensor meansincludes a surface which is exposed to said solution, said surfaceconforming to a segment of a sphere, and wherein said electrodescomprise first and second conductive areas positioned on said surface.

20. A chemical analyzer according to claim 19 wherein said conductiveareas have the shape of half circles and are positioned with theirstraight sides parallel and spaced apart.

21. In a chemical analyzing system for determing the concentration of acomponent in a sample, wherein said sample, upon being introduced intosolution with a reagent, reacts therewith causing an instantaneouschange in a characteristic of said solution or a component or a productof said reaction, the rate of the reaction being indicative of saidconcentration, such system comprising sensor means for monitoring saidcharacteristic, component or product and for producing a firstelectrical output signal proportional thereto, said change being sensedby said sensor means producing an abrupt change in said first outputsignal, and differentiator circuit means for producing a secondelectrical signal proportional to the time derivative of said firstsignal, said time derivative signal being indicative of saidconcentration of said component in said sample, the improvementcomprising:

timing means responsive to an abrupt change in said first signal forproducing a control signal, a characteristic of which changes after apredetermined, fixed time interval after said abrupt change in saidfirst signal; and

means responsive to said second output signal and operative upon theoccurrence of said change in said characteristic of said control signalfor determining the value of said second signal.

22. In a chemical analyzing system according to claim 21, theimprovement wherein said control signal is applied to saiddifferentiator circuit means for inhibiting the operation thereof duringsaid fixed time interval and wherein said determining means measures themaximum value of said second signal.

23. In a chemical anaylzing system according to claim 21, theimprovement wherein said determining means measures the instantaneousvalue of said second signal upon termination of said fixed timeinterval.

24. In a chemical analyzing system according to claim 21, theimprovement wherein said determining means measures the peak value ofsaid second signal after termination of said fixed time interval.

25. In a chemical analyzing system according to claim 21, theimprovement further comprising:

means for displaying the determined value of said second signal.

26. In a chemical anaylzing system according to claim 21, theimprovement wherein said timing means produces a second control signal,a characteristic of which changes before the termination of saidpredetermined fixed time interval, wherein said second control signal isapplied to said differentiator circuit means for inhibiting theoperation thereof during a first portion of said fixed time interval,and wherein said firstmentioned control signal is applied to saiddetermining means for inhibiting the operation thereof during said fixedtime interval.

27. In a chemical analyzing system according to claim 26, theimprovement wherein said determining means measures the instantaneousvalue of said second signal upon termination of said fixed timeinterval.

28. In a chemical analyzing system according to claim 26, theimprovement wherein said determining means measures the value of saidsecond signal at a known time after termination of said fixed timeinterval.

29. In a chemical analyzing system according to claim 21, theimprovement wherein said sensor means comprises first and secondconductive elements for monitoring the conductance of said solution,wherein said second electrical signal is proportional to the rate ofchange of conductance of said solution, and wherein said determiningmeans measures the value of said rate of change of conductance aftersaid predetermined, fixed time interval from introduction of said sampleinto said reagent.

30. In a chemical analyzing system according to claim 29, theimprovement wherein said conductive elements are substantially planarand are positioned in said sample coplanar to minimize capacitanceeffects on the measurement of conductance.

31. In a chemical analyzing system according to claim 29, theimprovement wherein said sensor means includes a surface which isexposed to said solution, said surface conforming to a segment of asphere, and wherein said conductive elements comprise first and secondconductive areas positioned on said surface.

32. In a chemical analyzing system according to claim 31, theimprovement wherein said conductive areas have the shape of half circleswhich are positioned with their straight sides parallel and spacedapart.

2. A method according to claim 1 wherein said fixed time interval islong enough to permit thorough mixing of said sample with said reagent.3. In a chemical analyzing system for determining the concentration of acomponent in a sample, wherein said sample, upon being introduced intosolution with a reagent, reacts therewith, the rate of the reactionbeing indicative of said concentration, such system comprising sensormeans for monitoring a characteristic of said solution or a component ora product of said reaction and for producing a first electrical outputsignal proportional thereto, and differentiator circuit means forproducing a second electrical signal propOrtional to the time derivativeof said first signal, said time derivative signal being indicative ofsaid concentration of said component in said sample, the improvementcomprising: means for inhibiting operation of said differentiatorcircuit means for a predetermined, fixed time interval from introductionof said sample into said reagent, said time interval being sufficient topermit thorough mixing of said sample with said reagent.
 4. In achemical analyzing system for determining the concentration of acomponent in a sample, wherein said sample, upon being introduced intosolution with a reagent, reacts therewith, the rate of the reactionbeing indicative of said concentration, such system comprising sensormeans for monitoring a characteristic of said solution or a component ora product of said reaction and for producing a first electrical outputsignal proportional thereto, differentiator circuit means for producinga second electrical signal proportional to the time derivative of saidfirst signal, said time derivative signal being indicative of saidconcentration of said component in said sample, and means responsive tosaid second signal for generating an output signal indicative of thevalue thereof, the improvement comprising: means for inhibiting thegeneration of said output signal for a predetermined, fixed timeinterval initiated automatically upon introduction of said sample intosaid reagent, said time interval being sufficient to eliminate theeffect of transients occurring upon introduction of said sample intosaid reagent.
 5. In a chemical analyzing system according to claim 4,the improvement wherein said inhibiting means inhibits the operation ofsaid differentiator circuit means during said fixed time interval.
 6. Ina chemical analyzing system according to claim 4, the improvementwherein said inhibiting means inhibits the operation of said generatingmeans during said fixed time interval.
 7. In a chemical analyzing systemfor determining the concentration of a component in a sample, whereinsaid sample, upon being introduced into solution with a reagent, reactstherewith, the rate of the reaction being indicative of saidconcentration, such system comprising sensor means for monitoring acharacteristic of said solution or a component or a product of saidreaction and for producing a first electrical output signal proportionalthereto, differentiator circuit means for producing a second electricalsignal proportional to the time derivative of said first signal, saidtime derivative signal being indicative of said concentration of saidcomponent in said sample, and means for measuring the value of said timederivative signal, the improvement comprising: means for inhibiting themeasurement of said time derivative signal for a predetermined, fixedtime interval from introduction of said sample into said reagent, saidtime interval being sufficient to eliminate the effect of transientsoccurring upon introduction of said sample into said reagent.
 8. In achemical analyzing system according to claim 7, the improvement whereinsaid fixed time interval is long enough to permit thorough mixing ofsaid sample with said reagent.
 9. In a chemical analyzing systemaccording to claim 7, the improvement wherein the rate of said reactionis not necessarily linear.
 10. In a chemical analyzing system accordingto claim 7, the improvement wherein said inhibiting means inhibits theoperation of said differentiator circuit means during said fixed timeinterval.
 11. In a chemical analyzing system according to claim 7, theimprovement wherein said inhibiting means inhibits the operation of saidmeasuring means during said fixed time interval.
 12. A chemical analyzercomprising: means for receiving a sample and a reagent; sensor meansoperatively associated with said receiving means for monitoring theconcentration of a component or product of the reaction between saidsample and said reagent and for producing a First output signalproportional to said concentration; differentiator circuit means coupledto said sensor means and responsive to said first output signal forproducing a second output signal proportional to the time derivative ofsaid first output signal and thus proportional to the time rate ofchange of concentration of said component or product; means coupled tosaid differentiator circuit means for measuring the value of said secondsignal; and means coupled to said sensor means for inhibiting themeasurement of the value of said second signal for a predetermined,fixed time interval from introduction of said sample and said reagentinto said receiving means, said time interval being sufficient toeliminate the effect of transients occurring upon introduction of saidsample and said reagent into said receiving means.
 13. A chemicalanalyzer according to claim 12 wherein said inhibiting means comprises:timing means responsive to an abrupt change in said first output signalfor producing a control signal, a characteristic of which changes aftersaid predetermined, fixed time interval; and wherein said measuringmeans comprises: means responsive to said second output signal andoperative upon the occurrence of said change in said characteristic ofsaid control signal for determining the value of said second outputsignal.
 14. A chemical analyzer according to claim 13 wherein saidcontrol signal is applied to said differentiator circuit means forinhibiting the operation thereof during said fixed time interval andwherein said measuring means measures the maximum value of said secondoutput signal.
 15. A chemical analyzer according to claim 13 whereinsaid timing means produces a second control signal, a characteristic ofwhich changes before the termination of said fixed time interval,wherein said second control signal is applied to said differentiatorcircuit means for inhibiting the operation thereof during a firstportion of said fixed time interval, and wherein said first-mentionedcontrol signal is applied to said measuring means for inhibiting theoperation thereof during said fixed time interval.
 16. A chemicalanalyzer according to claim 12 wherein said sensor means comprises:first and second electrodes extending into said receiving means formonitoring the conductance of the solution therein, wherein: said secondoutput signal is proportional to the rate of change of conductance ofsaid solution, and wherein: said measuring means measures the value ofsaid rate of change of conductance after said predetermined, fixed timeinterval.
 17. A chemical analyzer according to claim 16 furthercomprising: oscillator means operatively coupled to one of saidelectrodes of said sensor means, said oscillator means generating an ACoutput signal; and demodulator means operatively coupled to the other ofsaid electrodes of said sensor means, said demodulator means receivingan amplitude modulated signal and producing a DC signal which comprisessaid second output signal.
 18. A chemical analyzer according to claim 16wherein said sensor means includes a surface which is exposed to saidsolution, and wherein said electrodes comprise first and secondconductive areas positioned on said surface, said conductive areas beingspaced apart.
 19. A chemical analyzer according to claim 16 wherein saidsensor means includes a surface which is exposed to said solution, saidsurface conforming to a segment of a sphere, and wherein said electrodescomprise first and second conductive areas positioned on said surface.20. A chemical analyzer according to claim 19 wherein said conductiveareas have the shape of half circles and are positioned with theirstraight sides parallel and spaced apart.
 21. In a chemical analyzingsystem for determing the concentration of a component in a sample,wherein said sample, upon being introduced into solution with a reagent,reacts therewith causing an instantaneOus change in a characteristic ofsaid solution or a component or a product of said reaction, the rate ofthe reaction being indicative of said concentration, such systemcomprising sensor means for monitoring said characteristic, component orproduct and for producing a first electrical output signal proportionalthereto, said change being sensed by said sensor means producing anabrupt change in said first output signal, and differentiator circuitmeans for producing a second electrical signal proportional to the timederivative of said first signal, said time derivative signal beingindicative of said concentration of said component in said sample, theimprovement comprising: timing means responsive to an abrupt change insaid first signal for producing a control signal, a characteristic ofwhich changes after a predetermined, fixed time interval after saidabrupt change in said first signal; and means responsive to said secondoutput signal and operative upon the occurrence of said change in saidcharacteristic of said control signal for determining the value of saidsecond signal.
 22. In a chemical analyzing system according to claim 21,the improvement wherein said control signal is applied to saiddifferentiator circuit means for inhibiting the operation thereof duringsaid fixed time interval and wherein said determining means measures themaximum value of said second signal.
 23. In a chemical analyzing systemaccording to claim 21, the improvement wherein said determining meansmeasures the instantaneous value of said second signal upon terminationof said fixed time interval.
 24. In a chemical analyzing systemaccording to claim 21, the improvement wherein said determining meansmeasures the peak value of said second signal after termination of saidfixed time interval.
 25. In a chemical analyzing system according toclaim 21, the improvement further comprising: means for displaying thedetermined value of said second signal.
 26. In a chemical analyzingsystem according to claim 21, the improvement wherein said timing meansproduces a second control signal, a characteristic of which changesbefore the termination of said predetermined fixed time interval,wherein said second control signal is applied to said differentiatorcircuit means for inhibiting the operation thereof during a firstportion of said fixed time interval, and wherein said first-mentionedcontrol signal is applied to said determining means for inhibiting theoperation thereof during said fixed time interval.
 27. In a chemicalanalyzing system according to claim 26, the improvement wherein saiddetermining means measures the instantaneous value of said second signalupon termination of said fixed time interval.
 28. In a chemicalanalyzing system according to claim 26, the improvement wherein saiddetermining means measures the value of said second signal at a knowntime after termination of said fixed time interval.
 29. In a chemicalanalyzing system according to claim 21, the improvement wherein saidsensor means comprises first and second conductive elements formonitoring the conductance of said solution, wherein said secondelectrical signal is proportional to the rate of change of conductanceof said solution, and wherein said determining means measures the valueof said rate of change of conductance after said predetermined, fixedtime interval from introduction of said sample into said reagent.
 30. Ina chemical analyzing system according to claim 29, the improvementwherein said conductive elements are substantially planar and arepositioned in said sample coplanar to minimize capacitance effects onthe measurement of conductance.
 31. In a chemical analyzing systemaccording to claim 29, the improvement wherein said sensor meansincludes a surface which is exposed to said solution, said surfaceconforming to a segment of a sphere, and wherein said conductiveelements comprise first and second conductive areas positioned on saidsurface.
 32. In a chemical analyzing system according to claim 31, theimprovement wherein said conductive areas have the shape of half circleswhich are positioned with their straight sides parallel and spacedapart.